This invention relates to a semiconductor optical modulator, and more particularly to, an electroabsorption-type optical modulator in which the absorption of a light wave supplied is controlled by changing an intensity of electric field applied to a semiconductor optical absorption layer, and to a method for making the same.
Recently, with the development of high-speed and long-distance optical communication systems, there occurs a problem in conventional direct modulation systems with semiconductor lasers. Namely, in conventional semiconductor laser direct modulation systems, wavelength chirping occurs when the modulation is conducted, whereupon the waveform, after transmitting through an optical fiber, is deteriorated. This phenomenon is amplified with an increase in signal modulation frequency or transmission distance. Especially, in a system that uses a conventional 1.3 xcexcm zero-dispersion fiber, this problem is serious. Even if a light source with a wavelength band of 1.55 xcexcm that has a small fiber propagation loss is used to increase transmission distance, the transmission distance is limited by dispersion limitation due to chirping.
This problem can be solved by an external modulation system that modulates light emitted from a semiconductor laser by an external optical modulator other than the semiconductor laser while keeping the semiconductor laser emitting the light with a constant optical output. Therefore, external optical modulators have been increasingly developed. The external optical modulators can be composed of dielectric materials such as LiNbO3 or semiconductor materials such as InP, GaAs. In particular, semiconductor optical modulators are attractive since they can be integrated with optical elements such as a semiconductor laser, an optical amplifier or electronic circuits such as FET and can be easily miniaturized and operated with a reduced voltage.
Semiconductor optical modulators include absorption-type optical modulators in which the intensity modulation is conducted by varying an optical absorption coefficient based on an effect that the absorption edge is shifted to a long-wave end by applying an electric field, e.g., a Franz-Keldish effect in bulk semiconductor or a quantum confined Stark effect in multiquantum well(MQW), and Mach-Zehnder-type modulators which utilize a refractive index variation caused by an electrooptic effect(Pockels effect) or a quantum confined effect in bulk semiconductor.
In Mach-Zehnder-type modulators, in principle, chirping can be reduced to zero. However, since they have a interference-type structure, a simple linear waveguide structure as in absorption-type modulators cannot be obtained; therefore their fabrication and operation are complicated. On the other hand, in absorption-type modulators, wavelength chirping is much lower than that of semiconductor laser direct modulation systems, but it cannot be reduced to zero. In electroabsorption-type optical modulators, when an electric field corresponding to an electrical signal is applied to an optical absorption layer, the absorption coefficient for an incident light with a wavelength(xcexop) longer than the absorption-edge wavelength of the optical absorption layer of the modulator is increased to extinguish the incident light. FIG. 1 shows detuning xcex94xcex (a difference between the wavelength of an incident light and the absorption-edge wavelength of the optical absorption layer) and applied electric field dependencies of the absorption coefficient. By setting detuning to an appropriate value, good extinction characteristics; can be obtained. Thus, an external electrical signal can be converted into ON/OFF light state. In this case, the refractive index of the optical absorption layer to the incident light also varies as shown in FIG. 2. Namely, when the intensity of the electric field varies from zero to a value to conduct absorption modulation, the refractive index of the optical absorption layer also varies. The magnitude of chirping when conducting ON/OFF light state is proportional to the ratio xcex94n/xcex94xcex1 of a refractive index variation xcex94n to an absorption variation xcex94xcex1. Therefore, since xcex94nxe2x89xa70 and xcex94xcex1xe2x89xa70 are usually obtained, positive chirping occurs. However, if the difference between the wavelength of incident light, xcexop and the absorption-edge wavelength of the optical absorption layer, xcexg,i.e., detuning xcex94xcexop (=xcexopxe2x88x92xcexg) is decreased, xcex94nxe2x89xa60 and xcex94xcexxe2x89xa70 can be satisfied, therefore allowing negative chirping operation. But, since the absorption coefficient is increased, a sufficient light output cannot be obtained in the ON light state. Recently, it has been experimentally proved that the chirping can be reduced by superposing a signal electric field corresponding to an electrical signal after applying a constant electric field to an electroabsorption-type optical modulator. Such an electroabsorption-type optical modulator is expected to be a modulator for ultra-high speed and long distance optical communications in the future.
For example, such an electroabsorption-type optical modulator is reported in K. Yamada et al.,xe2x80x9cLow-Chirp, Low-Polarization Dependent Characteristics of Electroabsorption Optical Intensity Modulator with an InGaASP Bulkxe2x80x9d, The Institute of Electronics Information and Information Engineers(IEICE), 1995 General Meeting, annex paper vol.1, p.349(lecture No. C-349). In this report, it is described that fiber dispersion durability in 10 Gb/s transmission and 1.55 xcexcm wavelength can be enhanced by superposing an electrical signal after applying a constant bias to the modulator to overcome dispersion limitations due to transmission distance. Also, K. Morito et al.,xe2x80x9c10 Gb/s Transmission over 100 km of Standard Fiber with a Blue Chirpxe2x80x9d, IEICE, 1995 Electronics Society Meeting, annex paper vol.1, p.301(lecture No.C-301) reports an example of an electroabsorption optical modulator with a MQW structure, which is integrated with a DFB laser. In this report, it is described that dispersion durability in 10 Gb/s-100 km transmission can be enhanced by pre-applying a constant bias of 1.1 V to the modulator.
As described above, the pre-bias applying techniques where a constant bias is pre-applied to the electroabsorption-type optical modulator have merits that chirping can be reduced and limitations to transmission distance due to dispersion can be overcome. However, there occur problems that the emission power in the ON state of light is decreased and the extinction ratio is deteriorated when the pre-bias is applied. Furthermore, a drive circuit(bias T circuit) for applying a constant bias to an external electrical signal as shown in FIG. 3 is required.
Accordingly, it is an object of the invention to provide an electroabsorption-type optical modulator in which negative chirp during modulation can be obtained without pre-applying a constant bias and sufficient light emission power and good extinction ratio can be obtained.
It is a further object of the invention to provide a method for making such an electroabsorption-type optical modulator.
It is a still further object of the invention to provide an optical communication module which adopts such an electroabsorption-type optical modulator.
It is a still yet further object of the invention to provide an optical communication system which adopts such an electroabsorption-type optical modulator.
According to the invention, an electroabsorption-type optical modulator, comprises:
a semiconductor substrate; and
a semiconductor buffer layer, a semiconductor optical absorption layer and a semiconductor cladding layer which are layered in order on the semiconductor substrate;
wherein the absorption of a light wave supplied to an end of the semiconductor optical absorption layer is controlled by changing an intensity of electric field applied to the semiconductor optical absorption layer; and
the semiconductor optical absorption layer has a first region with absorption-edge wavelength shorter than that of second region of the semiconductor optical absorption layer and a voltage corresponding an external electrical signal is simultaneously applied to both the first and second regions of the semiconductor optical absorption layer, so that, to an incident light, a refractive index of the semiconductor optical absorption layer is decreased and an absorption coefficient of the semiconductor optical absorption layer is increased when an intensity of electric field applied to the semiconductor optical absorption layer corresponding to the external electrical signal is increased.
Also, according to the invention, an electroabsorption-type optical modulator, comprises:
a semiconductor substrate; and
a semiconductor buffer layer, a semiconductor optical absorption layer and a semiconductor cladding layer which are layered in order on the semiconductor substrate;
wherein the absorption of a light wave supplied to an end of the semiconductor optical absorption layer is controlled by changing an intensity of electric field applied to the semiconductor optical absorption layer; and
the semiconductor optical absorption layer has absorption-edge wavelengths which continuously vary along the direction of waveguiding an incident light from a short-wave side to a long-wave side or from a long-wave side to a short-wave side, so that, to an incident light, a refractive index of the semiconductor optical absorption layer is decreased and an absorption coefficient of the semiconductor optical absorption layer is increased when an intensity of electric field applied to the semiconductor optical absorption layer corresponding to the external electrical signal is increased.
Also, according to the invention, an electroabsorption-type optical modulator, comprises:
a semiconductor substrate; and
a semiconductor buffer layer, a semiconductor optical absorption layer and a semiconductor cladding layer which are layered in order on the semiconductor substrate;
wherein the absorption of a light wave supplied to an end of the semiconductor optical absorption layer is controlled by changing an intensity of electric field applied to the semiconductor optical absorption layer; and
the semiconductor optical absorption layer has absorption-edge wavelengths which vary stepwise along the direction of waveguiding an incident light, so that, to an incident light, a refractive index of the semiconductor optical absorption layer is decreased and an absorption coefficient of the semiconductor optical absorption layer is increased when an intensity of electric field applied to the semiconductor optical absorption layer corresponding to the external electrical signal is increased.
According to another aspect of the invention, a method for making a semiconductor optical modulator, comprises the steps of:
forming a first semiconductor buffer layer on a semiconductor substrate;
forming a first dielectric mask for selective growth on the first semiconductor buffer layer, the first dielectric mask including two regions in which a dielectric mask width is partially varied and a blank region sandwiched by the two regions;
forming a second semiconductor buffer layer, a semiconductor optical absorption layer and a semiconductor cladding layer in order on the blank region;
forming a second dielectric mask for selective growth which has a blank region broader than the blank region of the first dielectric mask;
forming a semiconductor cladding layer and a semiconductor capping layer in order on the blank region of the second dielectric mask for selective growth;
forming a first dielectric protective film over the entire top surface of the substrate;
forming a second dielectric protective film on the first dielectric protective film;
removing the first dielectric protective film and the second dielectric protective film until the semiconductor capping layer is exposed;
forming an electrode for applying an electric field to the semiconductor optical absorption layer on the semiconductor capping layer; and
removing the second dielectric protective film except under the electrode;
wherein the electrode on the semiconductor capping layer is formed all over the semiconductor optical absorption layer, so that a voltage corresponding to an external signal is uniformly applied to the semiconductor optical absorption layer.
According to a further aspect of the invention, an optical communication module, comprises:
the electroabsorption-type optical modulator defined above;
a first light-condensing means for optically coupling an input light to the semiconductor optical absorption layer of the electroabsorption-type optical modulator; and
a second light-condensing means for optically coupling a light output from the semiconductor optical absorption layer to an external optical fiber.
According to a further aspect of the invention, an optical communication system, comprises:
a transmitter which includes the electroabsorption-type optical modulator defined above;
means for externally waveguiding a light output from the transmitter; and
a receiver for receiving the light output from the waveguiding means.
In this invention, the semiconductor electroabsorption-type optical modulator has an optical absorption layer with a plurality of regions with different absorption-edge wavelengths along the direction of waveguiding a light, i.e., the optical absorption layer includes a region where a absorption-edge wavelength is shifted to a long-wave side. Therefore, when a uniform electric field is applied to the regions, as the entire modulator, the refractive index is varied to a negative direction and the absorption coefficient is increased. Also, the ratio of lengths of the short-wave side and the long-wave side of the absorption layer is properly determined so that the absorption coefficient of the entire absorption layer is sufficiently small when no bias is applied. Thus, negative chirp during a modulation can be obtained without pre-applying a constant bias and sufficient light emission power and good extinction ratio can be obtained.
To provide such a structure, this invention employs a bandgap control technique using a selective MOVPE(metal organic vapor phase epitaxy) method. According as each of the widths of dielectric mask regions on both sides of the blank region is partially varied, the absorption-edge wavelength of the optical absorption layer is partially varied. By using the crystal growth method, the optical absorption layer with partially different absorption-edge wavelengths can be formed in one growth step, i.e., it is not necessary to repeat etching and crystal growth for divided regions. Therefore, an electroabsorption-type optical modulator can be made with good uniformity, reproducibility and controllability.